Light olefins, defined herein as ethylene and propylene, serve as feeds for the production of numerous chemicals. Olefins traditionally are produced by petroleum cracking. Because of the limited supply and/or the high cost of petroleum sources, the cost of producing olefins from petroleum sources has increased steadily.
Alternative feedstocks for the production of light olefins are oxygenates, such as alcohols, particularly methanol, dimethyl ether, and ethanol. Alcohols may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for olefin production.
The catalysts used to promote the conversion of oxygenates to olefins are molecular sieve catalysts. Because ethylene and propylene are the most sought after products of such a reaction, research has focused on what catalysts are most selective to ethylene and/or propylene, and on methods for increasing the life and selectivity of the catalysts to ethylene and/or propylene.
The conversion of oxygenates to olefins in a hydrocarbon conversion apparatus (HCA) generates and deposits carbonaceous material (coke) on the molecular sieve catalysts used to catalyze the conversion process. Excessive accumulation of these carbonaceous deposits will interfere with the catalyst's ability to promote the reaction. In order to avoid unwanted build-up of coke on molecular sieve catalysts, the oxygenate to olefin process incorporates a second step comprising catalyst regeneration. During regeneration, the coke is at least partially removed from the catalyst by combustion with oxygen, which restores the catalytic activity of the catalyst. The regenerated catalyst then may be reused to catalyze the conversion of oxygenates to olefins.
Typically, oxygenate to olefin conversion and regeneration are conducted in separate vessels. The coked catalyst is continuously withdrawn from the reaction vessel used for conversion to a regeneration vessel and regenerated catalyst is continuously withdrawn from the regeneration vessel and returned to the reaction vessel for conversion.
Conventionally, in order to produce an increased volume of desired product or to form different products, multiple, complete and independent reactor systems with independent separation vessels were required. Each reactor in the multiple, complete and independent reactor systems had its own regeneration system and/or stripping system. With multiple regeneration and/or stripping systems comes an attendant multiplication of costs.
It is therefore desirable to reduce number of regeneration units and/or stripping units in order to reduce the tremendous costs associated with implementing a plurality of multiple, complete and independent reactor systems.